NAME

SYNOPSIS

DESCRIPTION

The
sigaction()
function allows the calling process to examine and/or specify the
action to be associated with a specific signal.
The argument
sig
specifies the signal; acceptable values are defined in
<signal.h>.

The structure
sigaction,
used to describe an action to be taken, is defined in the header
<signal.h>
to include at least the following members:

Member Type

Member Name

Description

void(*) (int)

sa_handler

SIG_DFL, SIG_IGN or pointer to a function.

sigset_t

sa_mask

Additional set of signals to be blocked during execution of signal-catching function.

int

sa_flags

Special flags to affect behaviour of signal.

void(*) (int, siginfo_t *, void *)

sa_sigaction

Signal-catching function.

If the argument
act
is not a null pointer, it points to a structure specifying the action to be
associated with the specified signal.
If the argument
oact
is not a null pointer, the action previously associated with the signal is
stored in the location pointed to by the argument
oact.
If the argument
act
is a null pointer, signal handling is unchanged; thus, the call can be used
to enquire about the current handling of a given signal.
The
sa_handler
field of the
sigaction
structure identifies the action to be associated with the
specified signal.
If the
sa_handler
field specifies a signal-catching function, the
sa_mask
field identifies a set of signals that will be added to the
process' signal mask before the signal-catching function is
invoked.
The SIGKILL and SIGSTOP signals will not be added to the signal
mask using this mechanism; this restriction will be enforced by
the system without causing an error to be indicated.

If the SA_SIGINFO flag (see below) is cleared in the
sa_flags
field of the
sigaction
structure, the
sa_handler
field identifies the action to be associated with the specified signal.
If the SA_SIGINFO flag is set in the
sa_flags
field, the
sa_sigaction
field specifies a signal-catching function.
If the SA_SIGINFO bit is cleared and the
sa_handler
field specifies a signal-catching function, or if the
SA_SIGINFO bit is set, the
sa_mask
field identifies a set of signals
that will be added to the signal mask of the thread
before the signal-catching function is invoked.

The
sa_flags
field can be used to modify the behaviour of the specified
signal.

The following flags, defined in the header
<signal.h>,
can be set in
sa_flags:

SA_NOCLDSTOP

Do not generate SIGCHLD when children stop.

SA_ONSTACK

If set and an alternate signal stack has been declared with
sigaltstack()
or
sigstack(),
the signal will be delivered to the calling process on that stack. Otherwise,
the signal will be delivered on the current stack.

SA_RESETHAND

If set, the disposition of the signal will be reset to SIG_DFL and the
SA_SIGINFO flag will be cleared on entry to the
signal handler.

Note:

SIGILL and SIGTRAP cannot be automatically reset
when delivered; the system silently enforces this restriction.

Otherwise, the disposition of the signal will not be modified on entry
to the signal handler.

In addition, if this flag is set,
sigaction()
behaves as if the SA_NODEFER flag were also set.

SA_RESTART

This flag affects the behaviour of interruptible functions; that is,
those specified to fail with
errno
set to [EINTR].
If set, and a function specified as interruptible is interrupted by this
signal, the function will restart and will not fail with [EINTR] unless
otherwise specified. If the flag is not set,
interruptible functions interrupted by this signal will fail with
errno
set to [EINTR].

SA_SIGINFO

If cleared and the signal is caught, the signal-catching function
will be entered as:

void func(int signo);

where signo is the only argument to the signal catching function. In
this case the sa_handler member must be used to describe the signal
catching function and the application must not modify the sa_sigaction
member.

If SA_SIGINFO is set and the signal is caught, the signal-catching function
will be entered as:

void func(int signo, siginfo_t *info, void *context);

where two additional arguments are passed to the signal catching function.
The second argument will point to an object of type
siginfo_t
explaining the reason why the signal was generated;
the third argument can be cast to a pointer to an object of type
ucontext_t
to refer to the receiving process' context that was interrupted when
the signal was delivered.
In this case the sa_sigaction member must be used to
describe the signal catching function and the application must not modify the
sa_handler member.

The si_signo member contains the system-generated signal number.

The si_errno member may contain implementation-dependent additional
error information; if non-zero, it contains an error number identifying the
condition that caused the signal to be generated.

The si_code member contains a code identifying the cause of the signal. If the
value of si_code is less than or equal to 0, then the signal was
generated by a process and si_pid and si_uid respectively indicate
the process ID and the real user ID of the sender.
The
<signal.h>
header description contains information about the signal
specific contents of the elements of the
siginfo_t
type.

SA_NOCLDWAIT

If set, and sig equals SIGCHLD,
child processes of the calling processes will not be transformed into zombie
processes
when they terminate. If the calling process subsequently waits for its
children, and the process has no unwaited for children that were transformed
into zombie processes, it will block until all of its children terminate, and
wait(),
wait3(),
waitid()
and
waitpid()
will fail and set
errno
to [ECHILD].
Otherwise, terminating child processes will be transformed into zombie
processes, unless SIGCHLD is set to SIG_IGN.

SA_NODEFER

If set and sig is caught,
sig will not be added to the process' signal mask on entry to the
signal handler unless it is included in sa_mask. Otherwise, sig
will always be added to the process' signal mask on entry to the signal
handler.

If
sig
is SIGCHLD and the SA_NOCLDSTOP flag is not set in
sa_flags,
and the implementation supports the SIGCHLD signal, then a
SIGCHLD signal will be generated for the calling process whenever
any of its child processes stop.
If
sig
is SIGCHLD and the SA_NOCLDSTOP flag is set in
sa_flags,
then the implementation will not generate a SIGCHLD signal in
this way.

When a signal is caught by a signal-catching function installed by
sigaction(),
a new signal mask is calculated and installed for the
duration of the signal-catching function (or until a call to
either
sigprocmask()
or
sigsuspend()
is made).
This mask is formed by taking the union of the current signal
mask and the value of the
sa_mask
for the signal being delivered
unless SA_NODEFER or SA_RESETHAND is set,
and then including the signal being delivered.
If and when the user's signal handler returns normally, the
original signal mask is restored.

Once an action is installed for a specific signal, it remains
installed until another action is explicitly requested (by
another call to
sigaction()),
until the SA_RESETHAND flag causes resetting of the handler,
or until one of the
exec
functions is called.

If the previous action for
sig
had been established by
signal(),
the values of the fields returned in the structure pointed to by
oact
are unspecified, and in particular
oact->sa_handler
is not necessarily the same value passed to
signal().
However, if a pointer to the same structure or a copy thereof is
passed to a subsequent call to
sigaction()
via the
act
argument, handling of the signal will be as if the original call to
signal()
were repeated.

If
sigaction()
fails, no new signal handler is installed.

It is unspecified whether an attempt to set the action for a signal
that cannot be caught or ignored to SIG_DFL is ignored or causes an
error to be returned with
errno
set to [EINVAL].

If SA_SIGINFO is not set in
sa_flags,
then the disposition of subsequent occurrences of
sig
when it is already pending is implementation-dependent;
the signal-catching function will be invoked with a single argument.
If the implementation supports the Realtime Signals Extension option,
and if SA_SIGINFO is set in
sa_flags,
then subsequent occurrences of
sig
generated by
sigqueue()
or as a result of any signal-generating function
that supports the specification of an application-defined value (when
sig
is already pending) will be queued in FIFO order until delivered
or accepted;
the signal-catching function will be invoked with three arguments.
The application specified value is passed
to the signal-catching function as the
si_value
member of the
siginfo_t
structure.

Signal Generation and Delivery

A signal is said to be
generated
for (or sent to) a process or thread
when the event that causes the signal
first occurs.
Examples of such events include detection of hardware faults,
timer expiration,
signals generated via the
sigevent
structure
and terminal activity, as well as invocations of
kill()
and
sigqueue()
functions.
In some circumstances, the same event generates signals for
multiple processes.

At the time of generation, a determination is made whether
the signal has been generated for the process
or for a specific thread within the process.
Signals which are generated by some action attributable to a
particular thread, such as a hardware fault,
are generated for the thread that caused the signal to be
generated.
Signals that are generated in association
with a process ID or process group ID
or an asynchronous event such as terminal activity
are generated for the process.

Each process has an action to be taken in response to each signal
defined by the system (see
Signal Actions
).
A signal is said to be
delivered
to a process when the appropriate action for the process and
signal is taken.
A signal is said to be
accepted
by a process when the signal is selected and returned by one of the
sigwait()
functions.

During the time between the generation of a signal and its
delivery or acceptance, the signal is said to be
pending.
Ordinarily, this interval cannot be detected by an application.
However, a signal can be
blocked
from delivery to a thread
If the action associated with a blocked signal is anything other
than to ignore the signal, and if that signal is generated for
the thread the signal will remain pending until it is unblocked,
it is accepted when it is selected and returned by a call to the
sigwait()
function, or the action associated with it is set to ignore the signal.
Signals generated for the process will be delivered to
exactly one of those threads
within the process which is in a call to a
sigwait()
function selecting that signal or has not blocked delivery of the
signal.
If there are no threads in a call to a
sigwait()
function selecting that signal, and
if all threads within the process block delivery of the signal,
the signal will remain pending on the process until a thread calls a
sigwait()
function selecting that signal,
a thread unblocks delivery of the signal,
or the action associated with the signal is set to ignore the signal.
If the action associated with a blocked signal is to ignore the
signal and if that signal is generated for the process, it is
unspecified whether the signal is discarded immediately upon
generation or remains pending.

Each thread
has a
signal mask
that defines the set of signals currently blocked from delivery
to it.
The signal mask for a thread
is initialised from that of its
parent
or creating thread, or from the corresponding thread in the parent
process if the thread was created as the result of a call to
fork().
The
sigaction(),
sigprocmask()
and
sigsuspend()
functions control the manipulation of the signal mask.

The determination of which action is taken in response to a
signal is made at the time the signal is delivered, allowing for
any changes since the time of generation.
This determination is independent of the means by which the
signal was originally generated.
If a subsequent occurrence of a pending signal is generated, it
is implementation-dependent as to whether the signal is delivered
or accepted more than once in circumstances
other than those
in which queueing is required under the Realtime Signals Extension option.
The order in which multiple, simultaneously pending signals
outside the range SIGRTMIN to SIGRTMAX are delivered to or accepted by
a process is unspecified.

When any stop signal (SIGSTOP, SIGTSTP, SIGTTIN, SIGTTOU) is
generated for a process, any pending SIGCONT signals for that
process will be discarded.
Conversely, when SIGCONT is generated for a process, all pending
stop signals for that process will be discarded.
When SIGCONT is generated for a process that is stopped, the
process will be continued, even if the SIGCONT signal is blocked
or ignored.
If SIGCONT is blocked and not ignored, it will remain pending
until it is either unblocked or a stop signal is generated for
the process.

An implementation will document any condition not specified by
this document under which the implementation generates signals.

Some signal-generating functions, such as
high-resolution timer expiration,
asynchronous I/O completion, interprocess message arrival, and the
sigqueue()
function,
support the specification of an application-defined value,
either explicitly as a parameter to the function or in a
sigevent structure parameter.
The sigevent structure is defined in
<signal.h>
and contains at least the following members:

Member Type

Member Name

Description

int

sigev_notify

Notification type

int

sigev_signo

Signal number

union sigval

sigev_value

Signal value

void(*)(unsigned sigval)

sigev_notify_function

Notification function

(pthread_attr_t*)

sigev_notify_attributes

Notification attributes

The
sigev_notify
member specifies the notification mechanism to use
when an asynchronous event occurs.
This document defines the following values for the
sigev_notify
member:

SIGEV_NONE

No asynchronous notification will be delivered
when the event of interest occurs.

SIGEV_SIGNAL

The signal specified in
sigev_signo
will be generated for the process when the event of interest occurs.
If the implementation supports the Realtime Signals Extension option
and if the SA_SIGINFO flag is set for that signal number, then the signal
will be queued to the process and the value specified in
sigev_value
will be the
si_value
component of the generated signal.
If SA_SIGINFO is not set for that signal number, it is unspecified whether
the signal is queued and what value, if any, is sent.

SIGEV_THREAD

A notification function will be called to perform notification.

An implementation may define additional notification mechanisms.

The
sigev_signo
member specifies the signal to be generated.
The
sigev_value
member is the application-defined value
to be passed to the signal-catching function
at the time of the signal delivery
or to be returned at signal acceptance as the
si_value
member of the
siginfo_t
structure.

The
sigval
union is defined in
<signal.h>
and contains at least the following members:

Member Type

Member Name

Description

int

sival_int

Integer signal value

void*

sival_ptr

Pointer signal value

The
sival_int
member is used when the application-defined value is of type
int;
the
sival_ptr
member is used when the application-defined value is a pointer.

If the Realtime Signals Extension option is supported:

When a signal is generated by the
sigqueue()
function
or any signal-generating function that supports the specification
of an application-defined value,
the signal will be marked pending and, if the SA_SIGINFO
flag is set for that signal, the signal will be queued to the process
along with the application-specified signal value.
Multiple occurrences of signals so generated are queued in FIFO
order.
It is unspecified whether signals so generated are queued when the
SA_SIGINFO flag is not set for that signal.

Signals generated by the
kill()
function or other events that cause signals to occur,
such as detection of hardware faults,
alarm()
timer expiration, or terminal activity,
and for which the implementation does not support queuing,
have no effect on signals already queued for the same signal
number.

When multiple unblocked signals, all in the range
SIGRTMIN to SIGRTMAX, are pending,
the behaviour will be as if
the implementation delivers the pending unblocked signal
with the lowest signal number within that range.
No other ordering of signal delivery is specified.

If, when a pending signal is delivered,
there are additional signals queued to that signal number,
the signal remains pending.
Otherwise, the pending indication is reset.

Multi-threaded programs can use an alternate event notification
mechanism:

When a notification is processed, and the
sigev_notify
member of the
sigevent
structure has the value SIGEV_THREAD, the function
sigev_notify_function
is called with parameter
sigev_value.

The function will be executed in an environment
as if it were the
start_routine
for a newly created thread with thread attributes specified by
sigev_notify_attributes.
If
sigev_notify_attributes
is NULL, the behaviour will as if the thread were created with the
detachstate
attribute set to PTHREAD_CREATE_DETACHED.
Supplying an attributes structure with a
detachstate
attribute of PTHREAD_CREATE_JOINABLE
results in undefined behaviour.
The signal mask of this thread is implementation-dependent.

Signal Actions

There are three types of action that can be associated with a
signal: SIG_DFL, SIG_IGN or a
pointer to a function.
Initially, all signals will be set to SIG_DFL or SIG_IGN prior to
entry of the
main()
routine (see the
exec
functions).
The actions prescribed by these values are as follows:

SIG_DFL - signal-specific default action

The default actions for the signals defined in this specification are
specified under
<signal.h>.
If the Realtime Signals Extension option is supported,
the default actions for the realtime signals in the range SIGRTMIN to
SIGRTMAX are to terminate the process abnormally.

If the default action is to stop the process, the execution of
that process is temporarily suspended.
When a process stops, a SIGCHLD signal will be generated for its
parent process, unless the parent process has set the
SA_NOCLDSTOP flag.
While a process is stopped, any additional signals that are sent
to the process will not be delivered until the process is
continued, except SIGKILL which always terminates the receiving
process.
A process that is a member of an orphaned process group will not
be allowed to stop in response to the SIGTSTP, SIGTTIN or
SIGTTOU signals.
In cases where delivery of one of these signals would stop such a
process, the signal will be discarded.

Setting a signal action to SIG_DFL for a signal that is pending,
and whose default action is to ignore the signal (for example, SIGCHLD),
will cause the pending signal to be discarded, whether or not it
is blocked.
If the Realtime Signals Extension option is supported,
any queued values pending will be discarded and the resources used to
queue them will be released and made available to queue other signals.

SIG_IGN - ignore signal

Delivery of the signal will have no effect on the process.
The behaviour of a process is undefined after it ignores a
SIGFPE, SIGILL, SIGSEGV
or SIGBUS
signal that was not generated by
kill(),
sigqueue()
or
raise().

The system will not allow the action for the signals SIGKILL or
SIGSTOP to be set to SIG_IGN.

Setting a signal action to SIG_IGN for a signal that is pending
will cause the pending signal to be discarded, whether or not it
is blocked.

If a process sets the action for the SIGCHLD signal to SIG_IGN,
the behaviour is unspecified,
except as specified below.
If the action for the SIGCHLD signal is set to SIG_IGN,
child processes of the calling processes will not be transformed into zombie
processes
when they terminate. If the calling process subsequently waits for its
children, and the process has no unwaited for children that were transformed
into zombie processes, it will block until all of its children terminate, and
wait(),
wait3(),
waitid()
and
waitpid()
will fail and set
errno
to [ECHILD].
If the Realtime Signals Extension option is supported,
any queued values pending will be discarded and the resources used to
queue them will be released and made available to queue other signals.

pointer to a function - catch signal

On delivery of the signal, the receiving process is to execute
the signal-catching function at the specified address.
After returning from the signal-catching function, the receiving
process will resume execution at the point at which it was
interrupted.
If the SA_SIGINFO
flag for the signal is cleared, the signal-catching function
will be entered as a C language function call as follows:

void func(int signo);

If the SA_SIGINFO flag for the signal is set, the signal-catching
function will be entered as a C language function call as follows:

void func(int signo, siginfo_t *info, void *context);

where
func
is the specified signal-catching function,
signo
is the signal number of the signal being delivered, and
info
is a pointer to a
siginfo_t
structure defined in
<signal.h>
containing at least the following member(s):

Member Type

Member Name

Description

int

si_signo

Signal number

int

si_code

Cause of the signal

union sigval

si_value

Signal value

The
si_signo
member contains the signal number.
This is the same as the
signo
parameter.
The
si_code
member contains a code identifying the cause of the signal.
The following values are defined for
si_code:

SI_USER

The signal was sent by the
kill()
function.
The implementation may set
si_code
to SI_USER if the signal was sent by the
raise()
or
abort()
functions
or any similar functions provided as implementation extensions.

The signal was generated by the expiration of a timer set by
timer_settime().

SI_ASYNCIO

The signal was generated by the completion of an
asynchronous I/O request.

SI_MESGQ

The signal was generated by the arrival of a message on an
empty message queue.

If the signal was not generated
by one of the functions or events listed above, the
si_code
will be set to an implementation-dependent value
that is not equal to any of the values defined above.

If the Realtime Signals Extension is supported,
and
si_code
is one of SI_QUEUE, SI_TIMER, SI_ASYNCIO, or SI_MESGQ, then
si_value
contains the application-specified signal value.
Otherwise, the contents of
si_value
are undefined.

The behaviour of a process is undefined after it returns normally
from a signal-catching function for a
SIGBUS,
SIGFPE, SIGILL or SIGSEGV signal that was not generated by
kill(),
sigqueue()
or
raise().

The system will not allow a process to catch the signals SIGKILL
and SIGSTOP.

If a process establishes a signal-catching function for the
SIGCHLD signal while it has a terminated child process for which
it has not waited, it is unspecified whether a SIGCHLD signal is
generated to indicate that child process.

When signal-catching functions are invoked asynchronously with process
execution, the behaviour of some of the functions defined by
this document is unspecified if they are called from a signal-catching
function.

The following table defines a set of interfaces that are either
reentrant or not interruptible by signals and are async-signal safe.
Therefore applications may invoke them, without
restriction, from signal-catching functions:

Realtime Interfaces

aio_error()

clock_gettime()

sigpause()

timer_getoverrun()

aio_return()

fdatasync()

sigqueue()

timer_gettime()

aio_suspend()

sem_post()

sigset()

timer_settime()

All functions not in the above table are considered to be unsafe
with respect to signals.
In the presence of signals, all functions defined by this specification
will behave as defined when called from or interrupted by a signal-catching
function, with a single exception: when a signal interrupts an unsafe
function and
the signal-catching function calls an unsafe function, the behaviour
is undefined.

>

>

When a signal is delivered to a thread,
if the action of that signal specifies termination, stop, or continue,
the entire process will be terminated, stopped, or continued,
respectively.

Signal Effects on Other Functions

Signals affect the behaviour of certain functions defined by this
specification if delivered to a process while it is executing such a
function.
If the action of the signal is to terminate the process, the
process will be terminated and the function will not return.
If the action of the signal is to stop the process, the process
will stop until continued or terminated.
Generation of a SIGCONT signal for the process causes the process
to be continued, and the original function will continue at the point
the process was stopped.
If the action of the signal is to invoke a signal-catching
function, the signal-catching function will be invoked; in this
case the original function is said to be
interrupted
by the signal.
If the signal-catching function executes a
return
statement,
the behaviour of the interrupted function will be as described
individually for that function.
Signals that are ignored will not affect the behaviour of any
function; signals that are blocked will not affect the behaviour
of any function until they are unblocked and then delivered,
except as specified for the
sigpending()
and the
sigwait()
functions.

The result of the use of
sigaction()
and a
sigwait()
function concurrently within a process on the same signal is
unspecified.

RETURN VALUE

Upon successful completion,
sigaction()
returns 0. Otherwise -1 is returned,
errno
is set to indicate the error and no new signal-catching function
will be installed.

ERRORS

The
sigaction()
function will fail if:

[EINVAL]

The
sig
argument is not a valid signal number or an attempt is made to
catch a signal that cannot be caught or ignore a signal that
cannot be ignored.

The
sigaction()
function may fail if:

[EINVAL]

An attempt was made to set the action to SIG_DFL for a signal
that cannot be caught or ignored (or both).

EXAMPLES

None.

APPLICATION USAGE

The
sigaction()
function supersedes the
signal()
interface, and should be used in preference. In particular,
sigaction()
and
signal()
should not be used in the same process to control the same signal.
The behaviour of reentrant interfaces, as defined in the
description,
is as specified by this specification, regardless of invocation from a
signal-catching function.
This is the only intended meaning of the statement that reentrant
interfaces may be used in signal-catching functions without
restrictions.
Applications must still consider all effects of such functions on
such things as data structures, files and process state.
In particular, application writers need to consider the
restrictions on interactions when interrupting
sleep()
and interactions among multiple handles for a file description.
The fact that any specific interface is listed as reentrant does
not necessarily mean that invocation of that interface from a
signal-catching function is recommended.

In order to prevent errors arising from interrupting
non-reentrant function calls, applications should protect calls
to these functions either by blocking the appropriate signals or
through the use of some programmatic semaphore (see
semget(),
sem_init(),
sem_open(),
and so on).
Note in particular that even the "safe" functions may modify
errno;
the signal-catching function, if not executing as an independent
thread, may want to save and restore its value.
Naturally, the same principles apply to the reentrancy of
application routines and asynchronous data access.
Note that
longjmp()
and
siglongjmp()
are not in the list of reentrant interfaces.
This is because the code executing after
longjmp()
and
siglongjmp()
can call any unsafe functions with the same danger as calling those
unsafe functions directly from the signal handler.
Applications that use
longjmp()
and
siglongjmp()
from within signal handlers require rigorous protection in order to be
portable.
Many of the other functions that are excluded from the list are
traditionally implemented using either
malloc()
or
free()
functions or the standard I/O library, both of which
traditionally use data structures in a non-reentrant manner.
Because any combination of different functions using a common
data structure can cause reentrancy problems, this document does not
define the behaviour when any unsafe
function is called in a signal handler that interrupts an unsafe function.

If the signal occurs other than as the result of calling
abort(),
kill()
or
raise(),
the behaviour is undefined if the signal handler calls
any function in the standard library other than one of the
functions listed in the table above
or refers to any object with static storage duration
other than by assigning a value to a static storage duration
variable of type
volatilesig_atomic_t.
Furthermore, if such a call fails,
the value of
errno
is indeterminate.

Usually, the signal is executed on the stack that was in effect before the
signal was delivered. An alternate stack may be specified to receive a subset
of the signals being caught.

When the signal handler returns, the receiving process will resume execution
at the point it was interrupted unless the signal handler makes other
arrangements. If
longjmp()
or
_longjmp()
is used to leave the signal handler, then the signal mask must be explicitly
restored by the process.

The ISO POSIX-1 standard defines the third argument of a signal handling function when
SA_SIGINFO is set as a void * instead of a ucontext_t *,
but without requiring type checking. New applications should explicitly cast
the third argument of the signal handling function to ucontext_t *.

The BSD optional four argument signal handling function is not supported by
this specification. The BSD declaration would be:

void handler(int sig, int code, struct sigcontext *scp,
char *addr);

where sig is the signal number, code is additional information on
certain signals, scp is a pointer to the sigcontext structure, and
addr is additional address information. Much the same information is
available in the objects pointed to by the second argument of the
signal handler specified when SA_SIGINFO is set.

FUTURE DIRECTIONS

The
fpathconf()
function is marked as an extension in the list of safe functions
because it is not included in the corresponding list in the ISO POSIX-1 standard,
but it is expected to be added in a future revision of that
standard.